1. Field of the Invention
The present invention relates to an optical method and apparatus for identifying or recognizing a pattern to be identified, by obtaining the correlation of images or patterns to be utilized in the field of optical data processing, and particularly relates to improved optical method and apparatus of recognizing a pattern or image to be identified, by presenting the correlation between a pattern to be identified and each of reference patterns.
2. Description of the Prior Art
There has been proposed a method of obtaining correlation between a pattern to be identified and each reference pattern of a group of reference patterns by displaying the pattern to be identified and the reference patterns on the same plane, and irradiating a coherent beam on such a plane to produce a first power spectrum of optical complex amplitude distribution to produce a first power spectrum image on another plane. Subsequent, irradiating coherent radiation to the resulting image based on the first power spectrum to produce a second power spectrum by optical way would thereby produce an optical output corresponding to the correlation between the pattern to be detected and the reference pattern.
The proposed method is useful in point of view of enabling optical processing in pseudo-real time even by using a spatial light-modulator with a relatively low resolution, and further, would improve an identifying ability when a plurality of reference images are presented by positive feed-back processing of correlation output between the pattern to be identified and each of the reference patterns.
FIG. 1 shows a prior art construction of an apparatus for obtaining correlation between a pattern to be identified and reference patterns of a group. This apparatus comprises a display 1 for displaying an input pattern; a first Fourier transformation means 2 for Fourier-transforming the input pattern displayed on the input display 1; a display 4 for presenting again the resulting first power spectrum obtained by the first Fourier transformation; a detector for detecting a second power spectrum obtained by a second Fourier transformation, and a feed back processor for modulating the intensity of the beam to irradiate the reference patterns formed on the input display plane 16 in accordance with the resulting correlation with the reference pattern.
The coherent beam 12 emitted from a coherent source 11 such as He-Ne laser is expanded by a beam expander 13, and passes through a beam splitter 14 and a first spatial light modulator 15 and irradiates an input display 16, in which the first spatial light-modulator 15 is a liquid crystal panel with arranged matrix electrode cells. The display 16 presents a pattern to be identified and reference patterns on the same liquid crystal panel, or alternatively a pattern to be identified is presented on a liquid crystal panel and reference patterns are presented on the same plane as that of the liquid crystal panel by a photographic film on which the reference patterns are recorded.
The input pattern is projected through a Fourier-transformation lens 21 to Fourier-transform spatially, thereby presenting the joint Fourier transformation pattern (power spectrum) on a screen 41. This pattern is imaged on a CCD camera 42, in which the pattern formed in accordance with the intensity of the output of the CCD camera 42 is fed through a processing and liquid crystal device circuit 43 into a liquid crystal panel 45 in the similar way. The pattern formed on the liquid crystal panel 45 corresponds to a power spectrum of the input pattern, which is irradiated by a beam 47 which has been formed by the reflection of the beam 12 at a beam splitter 14, and then, the pattern is Fourier transformed by a lens 51 to form a Fourier transformed pattern (power spectrum) on a screen 52. The resulting pattern formed on the screen 52 is a distribution of the intensity according to each correlation with each of the patterns, at the positions, corresponding to the positions of each of the input patterns, by centering to the optical axis thereof.
The distribution thereof is detected by a CCD camera 61, and then, the distribution of the transmittance of the spatial light-modulator 15 is modulated in accordance with the detected correlation by a processing and device circuit 62, so as to control the light-intensity distribution of the incident beam to each of the reference patterns formed on the display 16. In other words, the reference pattern with high correlation to the pattern to be identified is more strongly irradiated, and in contrast, the reference pattern with low correlation is more weakly irradiated, so that the loop processing in such fed-back will produce more accurate correlation and identification.
However, in such method of producing the correlation between the pattern to be identified and each of the reference patterns, when the number of the reference patterns is large, the additional output of the correlation between the reference patterns is presented in addition to the output of the correlation between the pattern to be identified and each of the reference patterns, such additional output will disturb the detection of the output of the correlation between the pattern to be identified and each of the reference patterns, and further, the first power spectrum obtained by the first Fourier transformation would contain more unnecessary information, so that the output of the correlation between the pattern to be identified and each of the reference patterns would contain more noise, or the S/N ratio would be lowered. Further, when the pattern to be identified is not exactly the same as the reference pattern, the interference pattern generated by the power spectrum of the reference patterns which would be presented at the position where the irradiation amount of the power spectrum of the pattern to be identified is substantially small and then, would interfere with correct recognition.